U.S. Can’t Track Tons of Weapons-Grade Uranium, Plutonium

U.S. Can’t Track Tons of Weapons-Grade Uranium, Plutonium

• BY NOAH SHACHTMAN
• 09.16.11
• 6:30 AM

President Obama has repeatedly said his top counterterrorism goal is to prevent terrorists from acquiring the building blocks to make nuclear or “dirty” bombs. In April of 2009, Obama announced a new international effort to “secure all vulnerable nuclear material around the world within four years.” Since then, the Department of Energy has dispatched scientists around the globe to collect hundreds of pounds of the stuff.

But according to a report from the Government Accountability Office (GAO), issued late last Friday afternoon to little fanfare, thousands of pounds of highly-enriched uranium and separated plutonium remain. American officials may never get a chance to ensure its security.

That’s because the U.S. can’t track or fully account for 5,900 pounds of “weapons usable” nuclear material that it once shipped overseas. Instead, U.S. officials have to rely on foreign governments’ assurances that the potentially cataclysmic stuff is safe. And when those officials occasionally visit the sites holding the nuclear material, nearly half the places “did not meet International Atomic Energy Agency security guidelines,” according to the GAO, Congress’ investigative arm.

“It’s amazing how completely cavalier the Department of Energy has been at tracking this. They’ve got nobody who worries about this on a day-to-day basis,” says Jeffrey Lewis, a nuclear weapons analyst at the Monterey Institute of International Studies (and occasional contributor to this blog).

The Energy Department, not surprisingly, has a different perspective. Foreign governments have pledged to report on the security of the their fissile material. There are international inspectors to keep those governments honest. And the GAO hasn’t reported that any uranium or plutonium has gone missing — just that certain guidelines may not have been yet.

“Between the International Atomic Energy Agency inspections and the reporting requirements, we think those safeguards are effective and internationally sanctioned,” Josh McConaha, a spokesman for the Energy Department’s National Nuclear Security Administration, tells Danger Room.

Starting in the Eisenhower administration, the U.S. sold 17,500 kilograms, or 38,5000 pounds, of fissile material overseas, mostly to help with civilian nuclear energy programs. Those sales came with conditions, however: countries had to keep the dangerous material safe; they couldn’t use it for weapons; and the U.S. had the option of taking back the radioactive stuff — someday, somehow.

But 12,400 of those 17,500 kilograms can’t be returned. It’s mostly in the hands — and reactors — of close allies like Germany, France, and Japan. 1,160 kilograms have been accounted for, and another 1,240 kg have been secured by the Energy Department’s “Global Threat Reduction Initiative,” an effort to covert nuclear power facilities from highly-enriched to low-enriched uranium, which is far less dangerous.

Still, don’t assume that just because the nuke material is at our friends’ houses means it is completely secure. One source familiar with the report’s development says, “If this was in some former Soviet republic, we’d be there in a heartbeat.” Some of America’s closest allies may be the ones with the poorest nuclear security precautions.

And there’s just one other problem. Subtracting all the nuke material that’s been accounted for and secured still leaves 2,700 kg — nearly three tons — outstanding. And that’s enough material to make dozens of nuclear weapons.

Where that uranium and plutonium is located — or, where it’s supposed to be located — the GAO report doesn’t say. That information was considered too sensitive to disclose in a public document, and was instead laid out in a classified report sent to Congress over the summer. But it’s worth noting that the U.S. currently has 27 so-called “Nuclear Cooperation Agreements” with 27 countries, from China to Ukraine to Colombia. America previously had similar deals with 11 other countries — including Israel, Pakistan, Venezuela, Vietnam, and Iran.

“Theoretically, we know [where the nuclear material is kept]. But we don’t have a good accounting of where it all is. We’re relying on them. We’re not, to coin a phrase, trusting but verifying,” the source says.

Occasionally, American inspectors will travel to these sites, to make sure these sites have the proper fences and surveillance gear needed to keep their nuclear material safe. The track record wasn’t particularly encouraging. Of those 55 visits conducted between 1994 and 2010, “physical protection teams found the sites met IAEA [International Atomic Energy Agency] security guidelines on 27 visits, did not meet IAEA security guidelines on 21 visits, and the results of 7 visits are unknown because the physical protection team was unable to assess the sites, or agency documentation was missing,” the report notes.

Partially, this alarming GAO report is an outgrowth of shifting standards. The U.S. is demanding more security and more accountability, to cope with a world in which terrorists have nuclear ambitions — and20 major atomic smugglers has been caught in the last two decades. Many countries haven’t caught up with those changes.

“The old way of doing business was: You bought it. We have some rights, but it’s fundamentally not our problem,” Lewis says. “Now, things are different.”

FISSILE MATERIALS IN THE US

Countries: United States

The United States is a nuclear weapon state member of the Non-Proliferation Treaty. In May 2010, the United States declared a stockpile of 5113 warheads as of the end of September 2009. This stockpile included almost 2000 strategic warheads deployed on about 800 missiles and bombers, 500 non-strategic warheads and 2,600 warheads in reserve. Additional 3500-4500 warheads are awaiting dismantlement. Taking into account the warheads in the dismantlement queue, the size of the U.S. weapon arsenal is estimated to be 9,400 nuclear warheads.

The current stock of fissile materials in the United States is estimated to include 94.8 tonnes of plutonium (80.7 tonnes of which is weapon-grade) and 686.6 tonnes of highly-enriched uranium (some of which is in irradiated naval fuel).  Of these amounts, 49.3 tonnes of separated plutonium and 194 tonnes of HEU have been declared as excess to military requirements. The United States has no separated civilian plutonium.

The United States is not producing fissile materials for weapons. Production of HEU for weapons ended in 1964. Additional HEU was produced for naval-reactor fuel through 1992. All U.S. production reactors were shut down in 1987.

Highly-enriched uranium

The United States was the first country to produce enriched uranium. During 1945-47, a little over a tonne of HEU was produced by electromagnetic separation at the Manhattan Project's Y-12 plant near Oak Ridge, Tennessee. Then the HEU production shifted to two large gaseous diffusion plants, one at Oak Ridge, Tennessee, and one at Portsmouth, Ohio.

The Oak Ridge gaseous diffusion plant produced HEU for weapons during 1945-1964 and thereafter produced only low-enriched uranium for nuclear power-plant fuel until 1985. The Portsmouth plant started production in 1956 and also produced HEU for weapons until 1964. Then it shifted to producing mostly low-enriched uranium for power-reactor fuel and HEU enriched to an average of 97.4 %, for naval-propulsion reactor fuel. There are several new commercial uranium enrichment facilities that are being built in the United States (seeFacilities: Uranium enrichment). None of them will be producing highly-enriched uranium.

Cumulatively, the United States acquired 751.9 tonnes of uranium-235 (835 tonnes of 90% HEU equivalent). As of the end of September 2004, about 180 tonnes of HEU had been consumed in nuclear reactor fuel, nuclear tests, transfers to foreign countries, and down-blending to low-enriched uranium (LEU). The measured HEU inventory was declared as 590.5 tonnes of U-235 in 686.6 tonnes of HEU as of September 2004 (656 tonnes 90% HEU equivalent). By the end of 2012, with about 141 tons of material either downblended or sent for downblending, the United States had a remaining stockpile of 604 tonnes of HEU.

Of the total HEU stock, about 100 tonnes had been fabricated into naval fuel that is to be disposed of in a geological repository after use. Another 159 tonnes have been designated for the naval-fuel reserve, and  20 tonnes have been reserved for use in space and research reactors.

Weapon plutonium

The first significant amounts of plutonium produced in the United States were used in the nuclear explosive that was tested in New Mexico on 16 July 1945 and then the bomb based on that design that was detonated over the Japanese city of Nagasaki on 9 August 1945. This plutonium was produced by the first three graphite-moderated, water-cooled reactors built on the U.S. Department of Energy's (DOE's) Hanford site on the Columbia River in Washington State. Later, an additional six such production reactors were built at Hanford and another five, moderated and cooled by heavy water, were built on the DOE's Savannah River Site in South Carolina. The primary mission of the Savannah River reactors was to produce tritium for U.S. nuclear weapons but they produced a great deal of weapon-grade plutonium as well.

U.S. production of weapon-grade plutonium peaked in the 1950's and early 1960's. During the 1960s, nine of the fourteen U.S. production reactors were shut down. Five continued to operate into the 1980s, primarily to produce tritium. All U.S. production reactors were finally shut down in 1987.

According to the official plutonium balance reselased by DoE in 2009, the United States produced and acquired 111.7 tonnes of plutonium. As of the end of September 2009, the United States had used and otherwise removed 14.0 tonnes of this plutonium. Measured inventory in 2009 was declared to be 95.4 tonnes, leaving 2.4 tonnes as inventory difference. Of the 95.4 tonnes, 81.3 tonnes is weapon-grade plutonium, 12.7 tonnes is fuel-grade, and 1.4 tonnes is power-reactor grade plutonium. These numbers do not include 3.8 tonnes of weapon-grade plutonium that had been disposed of as waste as of 2009.

In its 2011 INFCIRC/549 declaration, the United States reported that 61.5 tonnes of government owned plutonium has beed declared as excess for national security needs. This amount includes 44.7 tonnes of separated plutonium, 4.6 tonnes of plutonium in unirradiated MOX fuel, and less than 0.05 tonnes held in the fuel fabrication process. In addition, 7.8 tonnes of the plutonium declared as excess is in irradiated fuel and 4.4 tonnes was disposed of as waste.

Taking into account that 7.8 tonnes of the plutonium inventory is in irradiated fuel and the 4.4 tonnes has beed disposed of as waste, the amount of separated plutonium is 87.0 tonnes (95.4 tonnes declared in 2009 minus 7.8 tonnes and minus additional 0.6 tonnes disposed of as waste between 2009 and 2011). However, this amount includes contaminated material, residuses and other forms.  

Civilian plutonium

The United States has no separated civilian plutonium. At the end of 2011, an estimated 546 tonnes of plutonium was contained in spent fuel stored at civilian reactor sites and 12 tonnes of plutonium in spent fuel stored elsewhere. These 12 tonnes include the 7.8 tonnes of government owned plutonium that was declared as excess to national security needs that is accounted for in the weapon plutonium section.

Shpack Landfill (FUSRAP Site) Norton, MA (5 acre site)

Schpack Landfill Schpack Landfill
		Shpack Landfill (FUSRAP Site) Norton, MA (5 acre site) The Shpack Landfill is located about 65 kilometers (40 miles) southwest of Boston in the towns of Norton and Attleboro, Massachusetts. The landfill began operating in the 1960s as a landfill for both industrial and domestic wastes. The landfill was closed under court-order in the mid-1960s. In the late 1970s a concerned citizen who detected elevated radiation levels at the site contacted the Nuclear Regulatory Commission. It was confirmed that there was radioactivity present that was above acceptable limits and probably originated from activities performed by Texas Instruments (formerly known as M&C Nuclear, Inc.). The landfill contains wastes that are contaminated with high-enriched uranium, low-enriched uranium, natural uranium, depleted uranium, radium and various chemicals. Macroscopic amounts of high-enriched uranium and other radioactive materials were removed during a 1980�s survey by Oak Ridge personnel. The site was part of the Formerly Utilized Sites Remedial Action Program (FUSRAP). The FUSRAP Program was transferred to the United States Army Corps of Engineers (USACE) in 1997 in accordance with the Energy and Water Development Appropriations Act for FY 1998.

Congressman Joe Kennedy sponsored town hall style meeting for Former TI employees

BY STEPHEN PETERSON SUN CHRONICLE 

ATTLEBORO - More than 100 former Metals & Controls and Texas Instruments workers and family members attended a forum Friday night to continue to deal with a legacy from the Cold War.

The session, held at Bristol Community College on the site of the former Metals & Controls plant, was intended to inform those who gathered about federal compensation programs available for workers and families of workers who may have contracted cancer from working with radioactive materials.

Thousands worked at the industrial complex, which manufactured nuclear fuel for the Navy and other customers, from 1952 to 1967, and hundreds have been diagnosed with cancer.

While only a small part of the plant was used for nuclear manufacturing, the Attleboro site has been classified by the federal government as a former "atomic weapons employer." Fuel manufactured at the site was used in nuclear submarines and government research reactors.

U.S. Rep. Joseph Kennedy III, D-Newton, and the U.S. Labor Department spearheaded the forum, and there was also a representative of the National Institute of Occupational Safety and Health.

Lance Lanier from the Cleveland office of the Department of Labor detailed the Energy Employees Occupational Illness Compensation Program.

A program for contractors and subcontractors of the Department of Energy that pays up to $150,000 to victims of 22 cancers was created in 2000.

Larry Darcey, a former TI manager who lives in Rehoboth and who helped spark awareness of the cancer issue, said he runs across many ex-workers with multiple cancers.

Mathew Savastano, 82, of Attleboro, said he worked for the company from 1956 to 1966, beginning in his mid-20s, and began suffering from asthma at age 40. He said he has had blood clots in his lungs and several bouts of pneumonia and been "in and out of the hospital."

"I worked on nuclear reactors, did the wiring," Savastano said, adding he was in contact with radioactive enriched uranium and once broke out in a rash that led him to being written up in a medical journal. "I got all of these residual medical problems. It doesn't seem fair they only cover" cancer.

Another program that provides money for any condition linked to exposure to toxic substances went into effect in 2004, Lanier said.

While nuclear work at TI stopped in 1967, residual radiation is believed to have remained until 1997, when the site was cleaned of contamination, officials said. They said there was no monitoring of radiation levels during the nuclear years but there was afterward.

Lanier acknowledged many workers likely became sick from working with toxic metals and chemicals as well.

Texas Instruments sold its Attleboro-based business in 2005, and Darcey said he would like to see coverage extend up to that time.

"We will investigate to see what we can do to broaden some of these categories" for coverage, Kennedy said.

Kenneth Collins, 75, of Swansea, said he worked at the plant in the early 1960s and his father William worked from 1954 to 67, dying of mouth and nose cancer in 1988. "Why did the government wait so long?" Collins asked, highlighting the lack of medical records.

One woman said her father is suffering from bone cancer and wondered if the application would go through before he died. "If it is terminal, we try to speed up the claims," Lanier said.

William Bourque, 82, of Taunton, worked at the plant from 1953 to 1989, and was in contact with nuclear materials for eight years from the mid-50s to mid-60s. He said he once had a piece of uranium stuck in him for months. "I did every job in a highly contaminated area," Bourque said, adding he was "proud" to do the work for the U.S.

The Sun Chronicle documented in a series of reports that more than 300 former workers suffering from cancer received about $30 million in payments and medical benefits under the Labor Department compensation program.

However, the newspaper found little was done to reach out to ex-employees. Kennedy raised the issue at a congressional hearing and received a pledge by Texas Instruments to contact former workers, and the company sent out more than 2,000 letters about the compensation program.

Although a representative from TI was not available for Friday's forum, Lanier said his department has been getting good cooperation from the company. "That is very rare," he said, adding they have employment records.

"We have gotten quite a few claims the last few months," Lanier said.

For more information, workers and families are advised to visit the Department of Labor online, and/or call 1-800-941-3943 or Kennedy's Attleboro office representative Lisa Nelson at 508-431-1110

Where did the list of 22 "specified cancers" for the SEC come from?

Where did the list of 22 "specified cancers" for the SEC come from? 

Did NIOSH develop this list?

Neither NIOSH nor the Advisory Board on Radiation and Worker Health developed the list of 22 "specified cancers," and neither can modify the list. The list is part of the EEOICPA statute, and only Congress can modify the list of "specified cancers." The origin for the list of "specified cancers" began in the Radiation-Exposed Veterans Compensation Act (REVCA) of 1988. The Radiation-Exposed Veterans Compensation Act's original list included 13 cancers that were based on published reports from National Research Council Committees that identified elevated risks of contracting these cancers after exposure to radiation. This list of 13 cancers was then incorporated into the original enactment of the Radiation Exposure Compensation Act in 1990. In 2000, the Radiation Exposure Compensation Act was amended to incorporate a variety of changes, including the addition of 6 more specified cancers. The list of "specified cancers" for inclusion in the SEC is comprised of 22 cancers: 19 cancers specified in RECA; renal and bone cancers as specified in EEOICPA; and male and female breast cancers, which are considered separately under EEOICPA.

The following table shows a comparison of the "specified cancers" under the Radiation Exposure Compensation Act, the Radiation-Exposed Veterans Compensation Act, and EEOICPA.

Comparison of "Specified Cancer" Lists¹

Cancer site	REVCA and RECA (1990)	RECA (2000)2	EEOICPA
Bile duct	X	X	X
Bone			X
Brain		X	X
Breast (female and male)	X	X	X
Colon		X	X
Esophagus	X	X	X
Gall bladder	X	X	X
Kidney			X
Leukemia (exc. CLL)	X	X	X
Liver	X	X	X
Lung		X	X
Lymphoma (NHL)	X	X	X
Multiple myeloma	X	X	X
Ovary		X	X
Pancreas	X	X	X
Pharynx	X	X	X
Salivary gland		X	X
Small intestine	X	X	X
Stomach	X	X	X
Thyroid	X	X	X
Urinary bladder		X	X

¹This table is provided for comparison purposes only and does not include other requirements that might apply, such as latency period and primary and tumor status (i.e., primary or secondary).

²This list is specifically for those cancers covered by RECA under open- air nuclear testing.

Out in the Cold - America's Cold War Veterans

Out in the Cold - America's Cold War Veterans

They are America's Cold War veterans, who forged weapons from a fearsome energy source and bravely endured years of radiation for a country that pledged to take care of them. Instead, government loopholes and evasions are making sure those promises are never kept.

EEOICPA's purpose is to recognize that nuclear weapons workers with any of 22 kinds of cancers (among them breast, colon, bladder, brain, and non-Hodgkin's lymphoma) are likely to have gotten their illnesses on the job, and that poor record keeping or gross health-safety negligence make it difficult to know exactly who was exposed, and to what extent. EEOICPA says that former weapons workers who are "at least as likely as not" to have gotten cancer from radiation are entitled to medical benefits and a lump-sum payment of $150,000. The law is an antidote to the legal action that workers might otherwise have to take, at their own expense, if they believe they are entitled to worker's comp. A hundred and fifty grand's not exactly pay dirt for people who've drained their bank accounts, taken out loans, or gone bankrupt fighting cancer. But, if nothing else, the measure was a gesture of appreciation. EEOICPA let workers believe that the government's heart was in the right place.

Seven years after the law passed, the crowd of Rocky Flats workers at the Westin saw a government that was heartless. As far as the Flats brothers and sisters were concerned, their piece of star-spangled legislation had been designed with loopholes and engineered to fail them. By now they had learned that EEOICPA was undermined by "dose reconstruction," a procedure that seemed more black magic than sound science.

Dose reconstruction is the responsibility of the National Institute for Occupational Safety and Health, or NIOSH. To reconstruct a radiation dose, NIOSH digs up whatever it can about a claimant—urinalysis, nasal swab results, medical files, DOE "incident" reports, dosimetry records, and personal histories. And therein lies the problem: Like Judy and Tom, everyone has a story. The more time I spent talking to former Rocky Flats employees, the more anecdotes I heard about faulty dosimeters and dubious orders. Two former government officials insisted that medical records "disappeared" during the 1989 FBI raid. A former administrative assistant says she was ordered to illegally shred workers' medical records in the 1990s. Some people I spoke to were still shackled by the culture of secrecy; when I'd press them for details, they'd clam up. One man simply quit talking to me the moment I opened my notebook. Despite overwhelming consistencies among workers' stories of questionable health safety, only a fraction of what they say can be corroborated. It's their word. None of them had the foresight to build a case history while they were producing bombs, cleaning up an environmental disaster, and tending to their lives. And the government's dose reconstruction program dismisses almost any anecdote that a worker cannot prove.

When a claimant's records are missing or incomplete, NIOSH will use "coworker data"—records from a colleague who performed a similar job at a similar point in time. NIOSH also refers to a "site profile"—a multipage report that the agency has created for some of the 79 weapons facilities in the United States, that summarizes which parts of a site were most radioactive, and when. (Site profiles do not exist for all facilities.) In the end, the hard data get sent to the Department of Labor, plugged into a "matrix," and tallied to determine a figure known as "probability of causation," or POC. The POC is the claimant's final score; it informs Labor if the claimant was "at least as likely as not" to have gotten cancer due to work at a nuclear weapons facility. Claimants with a POC of 50 percent or higher are compensated. Claimants with a POC of 49.99 percent or lower are not.

Judy Padilla applied for compensation in August 2001—and waited nearly four years for a response. Her dose reconstruction score was 42.19; she was denied. She appealed the decision to the Department of Labor, explaining that she'd worked around ionizing radiation for the better part of 14 years, and that six of those years were spent chest-to-glove box, handling plutonium. Seven of her remaining eight years, she reminded the DOL, were spent working with thorium-equipped welding gear and completing tasks in the process areas. Like her coworkers, she'd seen or been near more fires and spills and accidents—some reported, some not—than she could count. She was a healthy, exercising nonsmoker, and two genetic tests showed no history of breast cancer in her family. Judy's appeal was denied.

Tom Haverty applied in July 2006. DOL still hasn't issued a decision, but a NIOSH worker recently told him that his prospects weren't good. Speaking by phone, the representative, Brian, told Tom he couldn't give specifics, but he indicated that Tom's score was less than 50. He said that Tom's final answer from DOL could take another eight months. Tom matter of factly stated that he'd likely be dead by then. Brian delivered news to Tom with the detached aplomb of an airline gate agent telling a passenger that his flight's been cancelled. It was clear that Brian had done this before.

Nearly 3,000 Rocky Flats workers have applied for compensation under this portion of EEOICPA; only 626, or 20 percent, have been paid. More than 69,000 weapons workers (or their families) across the country have submitted claims; at least one-third of them have been denied. The reason for their denials boils down to the dose reconstruction results—meaning they couldn't prove that they were "at least as likely as not" to have gotten cancer from radiation. They were given the burden of proof.

Larry Elliott oversees NIOSH's dose reconstruction program, and he defends his agency's work. "What most people don't understand is that dose reconstruction is an accepted scientific program to fill data gaps," he recently told me. "A high percentage of Rocky Flats workers have monitoring records, and NIOSH has those records." But, he went on, not all people were monitored. "We do not have individual monitoring records for every worker." He spoke of "unknown primaries" and "upper ranges." He assured me that dose reconstruction was set up to be as "claimant favorable" as possible.

Outside of NIOSH, it's tough to find anyone who supports the way the agency applies dose reconstruction. Richard Miller has worked as a senior policy analyst for the D.C. watchdog group Government Accountability Project and as a staff representative for DOE employees. Just last year, testifying before a House Judiciary Subcommittee, he said glove-box workers handling radiation at Rocky Flats (and other sites around the country) "were not adequately shielded for many years...[dosimeter] readings did not necessarily capture the neutron dose from leaky glove boxes, since the badges were not positioned near the parts of the glove box that leaked radiation." Tom Haverty's translation: "Radiation can blow up your skirt. It can radiate your skull. We wore dosimeters around our necks, not on our heads."

Even champions of the EEOICPA law acknowledge that the process of dose reconstruction is debatable. They point out that this particular brand of science was originally modeled to study large, unmonitored populations, like survivors of Hiroshima and Nagasaki, who were exposed to a single big blast, or atomic veterans who were involved in early weapons testing—not individuals who were exposed to low levels of radiation over long periods of time.

"When the bill was written, people on the Hill knew that any kind of science was imperfect—the law was even amended a few times to try to address that," Cindy Blackston, a former Judiciary staffer intimate with EEOICPA, recently told me. "But science is only as good as the perspective of the individual interpreting it. Some people within the system have interpreted the law so that claimants are placed on the defensive—which is exactly what the law was supposed to remedy. In many cases, the good intentions of EEOICPA have been abandoned."

Radiation Exposure and Cancer

The associations between radiation exposure and cancer are mostly based on populations exposed to relatively high levels of ionizing radiation (e.g., Japanese atomic bomb survivors and recipients of selected diagnostic or therapeutic medical procedures). Cancers associated with high dose exposure include leukemia, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers. Literature from the U.S. Department of Health and Human Services also suggests a possible association between ionizing radiation exposure and prostate, nasal cavity/sinus, pharyngeal and laryngeal, and pancreatic cancers.

Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, literature from the National Cancer Institute indicates that other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet) significantly contribute to many of these same diseases.

Although radiation may cause cancer at high doses and high dose rates, public health data do not absolutely establish the occurrence of cancer following exposure to low doses and dose rates — below about 10,000 mrem (100 mSv). Studies of occupational workers who are chronically exposed to low levels of radiation above normal background have shown no adverse biological effects. Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures.

A linear no-threshold (LNT) dose-response relationship is used to describe the relationship between radiation dose and the occurrence of cancer. This dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The U.S. Nuclear Regulatory Commission (NRC) accepts the LNT hypothesis as a conservative model for estimating radiation risk.